1. Technical Field
The present invention relates to a screen and an image display system.
2. Related Art
There have been known technologies for displaying video images from a projector or any other apparatus on a screen. In recent years, there has been a proposed technology for displaying video images by using a screen capable of switching its state between a transmission (transparent) state and a scatter state and projecting video image light on the screen that operates in the scatter state. Using the technology to set the screen to be transparent when no video image is projected reduces a feeling of spatial oppression produced by the screen not in use. A screen capable of switching its state between the transmission (transparent) state and the scatter state by using a liquid crystal layer has been proposed (JP-A-6-82748, for example) as an example of the screen described above.
As the technology for switching the state of a screen between the transmission (transparent) state and the scatter state by using a liquid crystal layer, there is a known polymer dispersion liquid crystal display apparatus in which a liquid crystal material is dispersed in a polymer (JP-A-10-36317, for example). The principle according to which a polymer dispersion liquid crystal display apparatus of this type operates uses the difference in refractive index between the liquid crystal material and the polymer. In a normal mode, the transmission (transparent) state is created by applying an electric field, and the scatter state is created by removing the electric field. In a reverse mode, the transmission (transparent) state is created by applying no electric field, and the scatter state is created by applying an electric field. When the polymer dispersion liquid crystal display device is used as a screen, and video image light is projected on the screen that operates in the scatter state, a desired image is displayed on the screen. In the scatter state, however, surrounding external light is also scattered and disadvantageously decreases the contrast of the displayed image.
To suppress the decrease in contrast described above, there is a known method for introducing a black dichromatic dye into a polymer dispersion liquid crystal material. The method prevents surrounding external light from being scattered and the contrast of an image from being degraded. In a screen (polymer dispersion liquid crystal display device) into which a dichromatic dye is introduced, however, the dichromatic dye does not respond sufficiently fast because the motion of the dichromatic dye is restricted in the polymer. The reason for this is that a dichromatic dye is driven based on a guest-host effect, but the motion of the dichromatic dye is restricted when a polymer is present.
When the problem described above occurs, the transmittance of the screen (polymer dispersion liquid crystal display device) in the transmissive (transparent) state decreases, resulting in a difficulty in achieving a sufficiently transparent state.